1. The Field of the Invention
The present invention generally relates to the field of joint repair surgery, such as reconstruction of the anterior cruciate ligament (ACL). More particularly, the invention relates to the field of interference screws used generally for fixation of soft tissue grafts, such as tibial affixation of soft tissue ACL grafts.
2. The Relevant Technology
Injuries to joints, specifically the knee, are quite common, particularly when one engages in vigorous sporting activities. A common injury is a rupture or tear of the anterior cruciate ligament (ACL), which is the primary ligament responsible for holding the knee joint together and which keeps it from slipping out of joint or dislocating. An unrepaired ruptured or torn ACL can cripple, and would most certainly limit physical activity of, the person suffering a ruptured or torn ACL. Absent reconstruction of the ACL, such injuries would likely be the end of professional sports careers and would prevent ordinary people from enjoying an active life involving sports and like recreation.
Improvements in surgical procedures have made ACL reconstruction procedures more successful and, hence, more common. One method for performing an ACL reconstruction procedure involves taking a tissue graft from another part of the body, such as a soft tissue graft (e.g., from the hamstrings), and attaching it at both ends through bone tunnels drilled through the two bones that make up the knee joint: the femur and the tibia. When secured in place, the tissue graft mimics and, hence, takes the place of, the ACL itself. This tissue graft holds the femur and tibia together to make the joint more stable, while simultaneously allowing for normal joint movements (i.e., flexion and extension).
Graft tension in ACL reconstruction has been recognized as an important factor in the clinical outcome of the ACL reconstruction procedure. In other words, grafts that are too loose may be unstable while grafts that are too tight may restrict motion of the knee.
One particular problem associated with soft tissue grafts, commonly referred to as the “bungee effect,” generally comprises excessive elasticity of the graft. The “bungee effect” occurs when the graft is affixed to the exterior surface of the bone, adjacent to the bone tunnel entrance, resulting in an effective graft length spanning not only the inside of the joint, but the length of the bone tunnel as well. The longer the graft, the more elastic the graft becomes. Accordingly, the “bungee effect” is accentuated with longer grafts. One method for minimizing the “bungee effect” is to utilize interference screws that shorten the effective length of the graft.
Interference screws are screwed into the bone tunnels through which the soft tissue graft passes, thereby causing the graft to be compressed against the surfaces of the bone tunnels with sufficient force for holding the graft in place. The interference screws shorten the effective length of the graft and minimize the “bungee effect” by affixing the soft tissue graft and preventing the graft from stretching inside of the bone tunnels.
Interference screws compress the graft against the exterior cortical bone regions, which are hard, as well as the interior cancellous bone regions, which are soft. To create sufficient pressures for holding the graft in place within the bone tunnels, particularly within the soft internal cancellous bone regions, interference screws are configured with diameters that are larger than the diameters of the bone tunnels. This enables the interference screw to compress the graft against the surface of the bone tunnel with sufficient force to hold the graft in place, where it ultimately bonds to the bone.
One problem with existing interference screws, however, is they often comprise bodies with substantially uniform diameters. This is a problem because it causes the interference screw to either exert too much pressure on the graft within the cancellous bone regions, thereby inhibiting healing and bonding of the graft to the bone, or alternatively, the interference screw exerts too little pressure on the graft within the cortical bone regions, thereby potentially resulting in insufficient holding strength for securing the graft in place.
Some existing interference screws are configured with a uniform taper spanning the entire length of the interference screws. These interference screws are also problematic. In particular, if the taper is very slight then the benefits of the taper are negligible and the pressures in the cancellous bone regions may remain too great. In contrast, a noticeable, or significant taper may result in insufficient compression of the graft for securing the graft in place, which is necessary for minimizing the “bungee effect” and for enabling the graft to bond with the bone.
Yet another problem with existing interference screws is that their trailing ends, or heads, are substantially flat and perpendicular with the body of the interference screw. This is generally a problem, as will be described below, because it is desirable for the interference screw to make complete circumferential contact with the cortical bone regions of the bone tunnel even though the bone tunnel is formed at an angle.
The strongest bone available for interference fixation of the graft is the cortical bone, which is located at and just beneath the surface of the bone. Accordingly, to maximize the screw fixation of the graft, the screw must optimally make complete circumferentially contact along the entire cortical bone region of the bone tunnel, including the exterior aperture of the bone tunnel. This is difficult, however, because the bone tunnels for tibial affixation of soft tissue grafts are formed at an angle, typically within the range of approximately 35° to approximately 40°. Due to the angle of the bone tunnel and the head configuration of existing screws, as described above, it is impossible for the screws to make complete circumferential contact along the entire cortical bone region of the bone tunnel when the screw is inserted entirely within the bone tunnel. Accordingly, if the trailing end of the screw is inserted completely within the bone tunnel, then fixation strength is compromised. Therefore, in order to obtain maximum cortical contact and fixation strength, existing interference screws can only be inserted within the bone tunnel to the point where the head of the interference screw starts to enter the bone tunnel, thereby resulting in a portion of the interference screw protruding out of the bone tunnel. This is a problem because protrusions can cause irritation and other problems for the patient, and therefore need to be removed with a cutting or grinding device, thereby increasing the risk, time, and cost associated with performing the ACL surgery.
In light of the foregoing, there is currently a need in the art of joint repair and soft tissue graft affixation to provide an improved interference screw that is configured to be inserted into obliquely aligned bone tunnels and for appropriately distributing within the bone tunnel the compressive forces that are used to hold the graft in place at the cancellous and cortical bone regions.